As a feedwater source, good quality condensate is preferred over other sources of makeup water because it does not create additional cost, thus avoiding the cost of softening, demineralizing, or heating other sources of makeup water. In particular, the heat content reduces fuel expenditures to generate the required steam load. Additional value exists in maximizing condensate return rates in paper mills that produce high absorbency products, such as tissue and towel products. In tissue- and towel-producing paper mills in particular, loading of various contaminants in the boiler feedwater creates problems in the system, particularly in the Yankee Dryer (i.e., the mechanical drum that forms the tissue or towel sheet). The nature of the Yankee Dryer produces substantial dissolved gases (e.g., CO2) in the condensate stream. The source of the majority of the dissolved gas is introduced to the boiler system through the makeup water, so maximizing the condensate return minimizes the dissolved gas loading in the system, which in turn makes the Yankee Dryer condensate easier to manage.
Several potential downsides exist to the return of condensate in the paper mill system. Papermaking is, at times, a batch process with the batches being defined as different grades of paper that are produced at different times on the same paper machine. Both paper machines and Yankee Dryers operate under steam pressure, so when downtime occurs, the steam flow is shut down and the components that use the steam pressure cool. To prevent complete steam collapse and a vacuum condition in such components, vacuum breaker valves open to allow air into the space previously occupied by steam. The oxygen fraction of the introduced ambient air causes corrosion on the hot metal surfaces, producing iron oxide. The longer the paper or tissue machine is offline, the more this chemical reaction can proceed, and the more iron oxide is generated. In addition, thermal contraction and expansion stresses caused during the cool down and start-up of the paper machine causes existing metal oxide layers to crack and exfoliate exposing more fresh metal surfaces than can undergo oxygen attack and corrode. When the machine returns to operation, the flow of steam and condensate flushes across the metal surfaces where the iron oxides have formed, stripping away the oxide particles. These particles form additional contaminants in the condensate return flow.
The condensate, as mentioned above, is the preferred source of boiler feedwater. But when condensate contains iron oxide particles, the boiler is put at risk for developing waterside deposits. Initially, these particles form a deposit layer that resists heat transfer, making the boiler less thermally efficient. Eventually, the deposit layer thickens causing the boiler tubes to overheat and fail, requiring the boiler to be taken off line for repair. Other, more subtle effects of iron deposits in the boiler waterside also occur. It is the nature of iron oxide deposits to be porous or permeable, allowing water to pass through and concentrate next to the hottest locations adjacent to the flame and causes steam to evaporate and concentrate boiler water salts. In some cases, this concentration proceeds to the point where under deposit corrosion (methods of removing such deposits are disclosed in U.S. patent Ser. No. 11/852,695, “Method and Device for Cleanup and Deposit Removal from Internal Hot Water System Surfaces,” currently pending) reactions take place, so that metals in the system (e.g., in the boiler tubes) dissolves via a mechanism referred to in the art as caustic corrosion or caustic embrittlement. The result is the same as for an overheat failure, the boiler must be taken off line for repair, at which time it is not available to generate steam for paper or tissue/towel drying, which curtails mill production and creates additional cost.
To minimize corrosion during both normal operating periods and downtime conditions, some mills feed chemical additives to the paper machine steam system. These chemicals are typically an oxygen scavenging, metal passivating/conditioning reducing agent mixed with a volatile alkaline amine, so that both the oxygen corrosion and acidic corrosion mechanisms are addressed. The current state of this technology is that there is no real time continuous monitoring and control capability to determine the corrosive nature of the paper machine condensate and make real time adjustments to the dosage of these chemical additives based on changing system conditions, even though it is known that the conditions are highly variable. A base feed dosage of the chemical additives are fed, and occasionally adjusted if grab samples or some other monitoring program is in place.
The papermaking industry has long sought to adapt equipment used for other water treatment purposes to provide continuous online information about condensate quality. For example, a control system that used turbidity to measure iron particles in condensate streams was accomplished by installing a turbidity meter on a flowing condensate sample in the area of the paper machine. This meter could then be used to activate a diversion valve when a sufficiently high concentration of particles was sensed. By analyzing grab samples for iron concentration, a correlation could be developed such that a preselected iron concentration could activate condensate dumping.
The use of turbidity measurement in managing the condensate iron concentration soon fell into disfavor. The equipment used to measure the sample's turbidity was designed for laboratory use, and could not withstand the conditions in the paper machine area. The turbidity meters required cooled sample for analysis, and the reliability of sample coolers in the process area proved to be a problem. Moreover, seasonal changes in the cooling water temperature that the mill provided were a problem. If the cooling water got too warm, the sample was not sufficiently cooled, and the optics used to detect the turbidity fogged over, making the results unreliable. Despite efforts to provide design solutions for these issues, the turbidity measurement of condensate iron concentrations as an online monitoring tool was abandoned after only a few years of use.
Standards for condensate monitoring today in the paper industry is represented by two approaches. First, in addition to iron particles, ionic solutions such as concentrated cooking liquors can contaminate condensate from time to time. As a solution, the industry has adopted a version of conductivity-based dumping or diversion system, whereby the detection of excessive conductivity in a sample will trigger the diversion system. Such a system is effective only to prevent significant ionic contamination events, but iron particles do not elevate the condensate conductivity sufficiently to provide detection. Second, manual testing of the condensate following downtime conditions is used to determine when the iron concentration is tolerable for use as boiler feedwater. By its nature, such manual testing is amenable to a substantial degree of variability making it difficult for precise control to be achieved. In most cases the condensate is dumped for a longer period than is technically required, creating additional costs for lost condensate and the concomitant need for increased makeup water and accompanying processing.
There thus exists a need in the papermaking industry to improve performance and efficiency of returning high quality condensate as a source of boiler feedwater. A particular need exists for improved online methods of optimizing injections of chemical additives into the boiler feedwater and steam/condensate system.